The invention relates to gaseous-based detectors for detection of ionizing radiation.
Gaseous-based ionizing radiation detectors, in general, are very attractive since they are cheap to manufacture and since they can employ gas multiplication to strongly amplify the signal amplitudes. A particular kind of gaseous-based ionizing radiation detector is the one, in which electrons released by interactions between photons and gas atoms can be extracted in a direction essentially perpendicular to the incident radiation.
Such a detector, of which a portion is, in a front view, illustrated in FIG. 1, comprises typically planar cathode 1, 2 and anode 3, 4 arrangements, respectively, and an ionizable gas arranged in the space 5 formed between the cathode and anode arrangements. The cathode 1, 2 and anode 3, 4 arrangements include each a metallic electrode plate or layer 1, 3 supported by a respective dielectric support 2, 4. The metallic electrode layers 1, 3 are facing each other and are separated by means of a dielectric spacer 6.
The detector is arranged such that a radiation beam 7 from a radiation source can enter the detector sideways between, and essentially parallel with, the cathode and the anode for ionizing the ionizable gas. Further, a voltage is typically applied between the metallic electrode layers 1, 3 for drifting electrons created during ionization of the ionizable gas towards the anode. The electric field caused by the applied voltage is indicated by field lines 8a-b in FIG. 1. Typically, the cathode is held at a negative potential, whereas the anode is grounded.
The voltage and the design of the detector electrodes may be adjusted to achieve a highly concentrated electric field such that the electrons are strongly accelerated and, by means of interactions with the gas, multiplied. Possibly, one or several grid or wire electrodes, which thus is/are electron permeable, may be arranged between the cathode 1, 2 and anode 3, 4 arrangements and may be held at electric potential(s) to achieve suitable amplification.
A read-out arrangement (not illustrated) is typically arranged in connection to the anode for detecting the charge induced by the multiplied electrons.
Due to the strong electric fields created in the gaseous-based ionizing radiation detector, however, spark discharges can occur in the gas, and these can block the detector for a period of time, and can also be harmful for the detector and particularly for electronics thereof. Such spark discharges are particularly probable to occur in high amplification detectors along surfaces of the dielectric spacer 6, which face the space 5 between the cathode and anode arrangements, where the electric field lines 8b are essentially parallel with the surface.
In order to reduce the occurrence of spark discharges the detectors are typically formed with a distance d1 between the outer edges of the conductive cathode and anode layers and the dielectric spacer 6 to define an outer gas volume 77 delimited by the inner surface 9 of the spacer 6, the uncovered electrode substrates 2, 4 outside the electrode layers 1, 3, and the space 5 between the electrode layers 1, 3. Hereby, a longer path along dielectric surfaces between the outer edges of the conductive cathode and anode layers is obtained, and thus the electric field strength is reduced at these surfaces, particularly at surface 9 of the dielectric spacer 6. The longer the distance d1 is, the better spark discharge protection is obtained, but to the cost of large and bulky detectors. In some applications this is not acceptable.
Alternatively, or as a complement, the surface 9 of the dielectric spacer 6 may be made rough or irregular to thereby prevent the formation of streamers at the surface 9. However, a structure of such kind having an effective protection against the occurrence of sparks would be difficult and/or expensive to manufacture.
A main object of the invention is to provide a detector for detection of ionizing radiation wherein problems caused by spark discharges are reduced or even eliminated.
In this respect there is a particular object to provide such a detector, which is compact and thus suitable to be used in cramped spaces or in a stacked multi-detector configuration.
A further object of the invention is to provide such a detector, which is reliable, accurate, inexpensive, and which has a long lifetime.
These objects, among others, are attained by detectors as claimed in the appended claims.
There are many theories about discharge formation in gases, and especially discharges along dielectric surfaces. The most common explanation is that an electrical discharge is preceded by a streamer formation. A streamer is string of plasma, i.e. electrical charges accumulated in the gas or along a surface. The electrical resistance of the streamer is significantly lower than the resistance of the gas, or of the surface of a dielectric. If a strong electric field is present close the streamer an electrical discharge can occur along the streamer more easily than through the gas itself, or along a dielectric surface.
In the case of the prior art illustrated in FIG. 1, ionizing radiation entering the outer gas volume (reference numeral 77 in FIG. 1) outside the peripheries of the electrode layers will ionize the gas liberating electrons and positive ions. These charges will follow the electric field lines and accumulate on the dielectric surface of the anode and cathode substrates facing the outer gas volume. This accumulation of charges is the initial formation of a streamer. As more charges are added, the charges tend to follow the field lines and creep up along the surface of the spacer facing the outer volume. As the streamer migrate, eventually it will form a short circuit between the anode and the cathode resulting in an electrical discharge along the streamer.
The inventors have found that by modifying the electric field configuration in the outer volume, and especially along the dielectric surfaces forming the boundaries of the outer volume, the probability of an electrical discharge can be significantly reduced. Field configurations where most of the electric field lines are perpendicular to the dielectric surfaces rather than parallel are especially beneficial for suppressing discharge formation.
Further characteristics of the invention, and advantages thereof, will be evident from the detailed description of preferred embodiments of the present invention given hereinafter and the accompanying FIGS. 2-6, which are given by way of illustration only, and thus are not limitative of the present invention.